19 



(iium-sized iudividuals being usually a little curved upward toward the 

 posterior end ; beaks small, nearly terminal, rounded on the umbones 

 and very slightly incurved; posterior extremity truncate, in the older 

 specimens rather squarely so below, and rounding forward to the extrem- 

 ity of the hinge-line above. In the smaller individuals it is narrower, 

 somewhat sloping from above or obliquely truncate. Anterior end very 

 slightly prolonged below the middle of the height and excavated beneath 

 the beaiis. A narrow linear escutcheon of considerable length is seen on 

 many of the separated valves. Anterior muscular impression rather 

 large and strongly impressed, as shown by the strong markings on 

 internal casts; i)osterior impression not distinguishable; pallial line 

 faint, and the space within the limits often marked by radiating lines as 

 in the recent forms of the genus. 



■ Surface of the valves ventricose, with a distinctly angular umbonal 

 slope in the larger specimen, but often obtusely rounded in the smaller 

 ones. A broad but rather faint depression runs obliquely backward 

 from the beak to the middle of the basal border on many individuals, 

 but is not always present. Surface of the shell marked by irregular 

 concentric lines of growth parallel to the margin of the valve. 



The species is closely allied to a form'described by Mr. Meek (in MS.) 

 under the name of Trepezium {Pachymya) trunoata, but is proportionally 

 very much more elongated than the specimens which are so labeled in 

 the collection at the Smithsonian Institution. The shells have been 

 exceedingly abundant in certain ln.yers, being densely packed together, 

 so that it is impossible to separate the individuals. They are quite vari- 

 able in form at different stages of growth, the younger shells being 

 transversely elongate-ovate, narrowed and rounded, or very obliquely 

 truncate from above posteriorly, and becoming almost squarely truncate 

 behind in the older specimens, the slight sinuosity of the basal margin 

 being hardly noticeable until they attain to near their adult size. 



Formation and locality.-^ln a hard and somewhat siliceous limestone of 

 reddish color, at a level of 350 feet above the Triassic beds east of Belle 

 Fourche, Black Hills, Dakota. 



TRAPBZItJM SUBEQUALTS, «. Sp. 



Plate 5, figa. 5-8. 



Shell small, transversely elongate-subelliptical, about twice and a half 

 as long as high, or nearly so. Valves somewhat ventricose, wiih large 

 tumid beaks situated a little nearest to the anterior end, and projecting 

 above the general line of the hinge; hinge-line, posterior to the beaks, 

 about half as long as the entire length of the shell, straight, but gradu- 

 ally declining to the extremity ; posterior end of the shell very obliquely 

 truncate, being much the longest below the middle of the posterobasal 

 angle ; anterior end shorter, slightly contracted in front of the beaks, 

 but rounded at the extremity and broader than the opposite end; basal 



